Coombs’ Test – Part 2 – Indirect, Anti-globulin indirect test
- This is done on mother serum.
- To diagnose the presence of antibodies in the pregnant mother serum.
- To diagnose autoimmune hemolytic anemia.
- To find the red cell sensitization by drugs.
- This should be done on fresh serum.
- Some of the drugs give false-positive results are cephalosporin, antiarrhythmic, insulin, methyldopa, Dilantin, sulphonamides, and tetracycline.
- The indirect Coombs’ test detects the circulating antibodies in the serum, these are the free antibodies.
- The major purpose of this test is to detect if the recipient or the patient has serum antibodies other than the ABO / Rh system to RBC before receiving the blood transfusion.
- This is the test for screening purposes in routine blood transfusion.
- This test can diagnose other agglutinins like cold agglutinins which are usually associated with Mycoplasmal infection.
- Circulating antibodies against RBC may be seen in the Rh-negative mother who is carrying an Rh-positive fetus.
- There is a mixing of the feto-maternal blood during the first delivery.
- Fetal blood Rh-Positive when goes into mother circulation will sensitize the mother and there is the formation of IgG type of antibodies.
- These antibodies can cross the placental barrier and enters the fetal circulation.
- In the second pregnancy, there is a Type II cytotoxic reaction which will destroy the fetal RBCs and give rise to hemolytic anemia.
- Coombs’ serum is prepared by injecting IgG immunoglobulin (antibody) to the rabbit, then from his serum anti-IgG antibody is separated, which is called Coombs serum.
- This test is done in the serum.
- Add recipient serum (patients) to donor RBC (known blood O) in a test tube.
- Incubate and then wash the RBC three times with saline.
- Now add Coombs’ serum and again centrifuge, then see for agglutination.
- If the test is positive then you will see clumping or agglutination.
- Negative and there is no agglutination.
A positive test indicates:
- Incompatible blood transfusion.
- Mother has anti-Rh antibodies.
- Erythroblastosis fetalis.
- Acquired hemolytic anemia.
- due to drugs and cold antibody.